This invention relates to a deflection scanning optical system wherein a polygonal mirror or the like deflects a laser beam to cause the deflected laser beam to scan a surface to be scanned and, more particularly, to a mirror for use in the scanning optical system to direct the laser beam toward a detecting element for detecting a scanning position of the laser beam with respect to the surface to be scanned, in order to obtain a horizontal synchronizing signal.
Recently, various apparatuses have been developed which employ an optical system wherein a polygonal mirror deflects a laser beam to cause the deflected laser beam to scan a surface to be scanned. Such apparatuses include, for example, a laser beam printer wherein a laser beam is used to scan a surface of a photoconductive drum along an axis thereof to carry out a main scanning, and the photoconductive drum is rotated to carry out an auxiliary scanning In this laser beam printer, an electrophotographic system is utilized to carry out printing The above-described apparatuses also include a laser photo-plotter wherein a laser beam is used to scan a photosensitive material surface to draw a pattern on an original plate of a printed circuit board or the like. In such laser beam printer, laser photo-plotter or the like, it is desirable to reduce the size and weight of the entire apparatus as far as possible, from the viewpoint of installation space, manufacturing cost, operability and the like of the apparatus.
On the other hand, if it is supposed that the wavelength of a laser beam is constant, the larger a numerical aperture NA of a condenser lens, the more reduced in diameter can the beam waist be. In other words, if the focal length of the condenser lens is the same, the larger the diameter of the condenser lens, the more reduced in diameter can the beam waist be. In this manner, if the beam waist is reduced in diameter, an allowable depth is lengthened so that more precise drawing is made possible. In order to raise the NA, however, it is required to increase the diameter of the condenser lens or to shorten the focal length thereof. This results in such problems that the manufacturing cost of the lens increases and an aberration of the optical system increases. It is therefore desirable to lengthen the laser beam optical path, i.e., the focal length of the condenser lens as far as possible.
For the reasons discussed above, the laser beam optical path is refracted repeatedly in an attempt to secure the requisite optical path length and to reduce the size of the apparatus, thereby making the above requirements compatible with each other.
In addition, in the above-mentioned scanning optical system, in order to control the writing timing of the laser beam, a reflecting mirror for exclusive use in such controlling is arranged at a predetermined position on the scanning path. A detecting element or light receiving element is so arranged as to receive the laser beam reflected by the reflecting mirror for detecting passage of the laser beam, i.e., the scanning position of the laser beam, to thereby generate a signal serving as a horizontal synchronizing signal.
However, the above-described conventional arrangement requires a multiplicity of reflecting mirrors for refracting the laser beam optical path, causing an increase in the cost of the component parts. Further, since the reflecting mirrors must be installed with high accuracy, considerable time is required for assembling and adjustment of the reflecting mirrors, resulting in an increase of the manufacturing cost.